Repeated treatment with amitriptyline or electroconvulsive shock does not affect thyrotropin releasing hormone receptors in discrete rat brain structures

Effects of pentylenetetrazole-induced kindling on thyrotropin-releasing hormone biosynthesis and receptors in rat brain

It has been postulated that changes in thyrotropin-releasing hormone biosynthesis may be involved in the mechanism of kindling--an animal model of epileptogenesis. To test this hypothesis, a time-course study was carried out to investigate the effects of pentylenetetrazole kindling (40 mg/kg i.p., daily for eight days) on the expression of gene coding for preprothyrotropin-releasing hormone, the thyrotropin-releasing hormone tissue level and thyrotropin-releasing hormone receptor parameters in rat brain. As shown by an in situ hybridization study, a single, convulsant dose of pentylenetetrazole (70 mg/kg i.p.) increased the preprothyrotropin-releasing hormone messenger RNA level in the dentate gyrus of the hippocampal formation and piriform cortex after 3 h and, to a greater extent, after 24 h. Those changes were accompanied with increases in the thyrotropin-releasing hormone level in the striatum, hippocampus, amygdala and piriform cortex. Seven days after single pentylenetetrazole administration, the thyrotropin-releasing hormone level was still significantly elevated in the piriform cortex and striatum. Acute pentylenetetrazole decreased the density (Bmax) of thyrotropin-releasing hormone receptors in the striatum after 3 and 24 h, and increased that density in the piriform cortex and amygdala after 24 h and seven days, respectively. The thyrotropin-releasing hormone receptor affinity (Kd) was decreased in the striatum and increased in the amygdala after only 3 h. Kindled rats showed a moderate increase in the preprothyrotropin-releasing hormone messenger RNA content in the dentate gyrus of the hippocampal formation and piriform cortex after 3 and 24 h; however, a significant decrease in those parameters was found after 14 days. After 3 and 24 h, pentylenetetrazole kindling also elevated the thyrotropin-releasing hormone content in the hippocampus, piriform cortex, and striatum (in the latter structure after 24 h only), whereas in the septum the thyrotropin-releasing hormone level was decreased. After seven days, the thyrotropin-releasing hormone level was still elevated in the hippocampus and piriform cortex of kindled rats, but after 14 days it was significantly lowered in the hippocampus. The kindled rats also showed a significant decrease in the density (Bmax) of thyrotropin-releasing hormone receptors in the striatum (after 24 h, seven and 14 days), and an increase in the piriform cortex (after seven days). The thyrotropin-releasing hormone receptor affinity (Kd) value was increased in the hippocampus after seven and 14 days, and in the piriform cortex after seven days. These results indicate that pentylenetetrazole kindling induces long-lasting alterations in the thyrotropin-releasing hormone biosynthesis and thyrotropin-releasing hormone receptor affinity in discrete regions of rat brain. These region-specific changes, in particular down-regulation of the thyrotropin-releasing hormone biosynthesis in the hippocampus, may be involved in chronic neuronal hyperexcitability associated with kindling.

Effects of repeated cocaine administration on the thyrotropin-releasing hormone level and receptors in the rat brain

The effects of single and repeated administration of cocaine on the thyrotropin-releasing hormone (TRH) level and receptors in discrete rat brain structures were evaluated. Male Wistar rats received saline or cocaine (15 mg/kg i.p., once an hour within 3 h, for 8 days). The animals were killed by decapitation at 45 min and 72 h (chronic group only) after the last injection. A radioimmunoassay (RIA) study showed that a single dose of cocaine increased the TRH level in the striatum by 68%, but had no significant effect on the peptide content in the nucleus accumbens, hippocampus, amygdala, septum, hypothalamus, frontal and prefrontal cortex at 45 min after the drug injection. Repeated administration of cocaine increased the TRH level in the striatum by 89% at 45 min, and in the hippocampus by 26% at 72 h after the last dose. No changes in the TRH level were found in other brain structures. In vitro cocaine (10(-6)-10(-4) M) inhibited the K(+)-stimulated release in a concentration-dependent manner, but had no effect on the basal release of TRH from the striatum and nucleus accumbens of naive rats. Acute cocaine decreased the Bmax of TRH receptors in the striatum, but had no effect on the density and affinity of TRH receptors in other brain regions. Repeated administration of cocaine evoked a long-lasting decrease in the Bmax of TRH receptors in the striatum (by c. 30%), whereas an increase in that parameter was observed in the frontal cortex. The Bmax and affinity of TRH receptors following repeated cocaine remained unchanged in the nucleus accumbens. The results obtained indicate that cocaine affects the TRH system mainly in the striatum, and to a lesser extent in the nucleus accumbens, cortex and hippocampus. Furthermore, the above changes do not resemble those induced by amphetamine, which points to certain differences in adaptation of the TRH neuronal system to these psychostimulants. On the other hand, the increase in the hippocampal TRH level during both chronic cocaine and morphine withdrawal is a common feature of the mechanism of dependence on these drugs.

The effects of single and repeated morphine administration on the level of thyrotropin-releasing hormone and its receptors in the rat brain

Effects of single (20 mg/kg i.p.), and repeated morphine (increasing doses: from 20 to 100 mg/kg/day i.p., twice daily for 10 days) administration on the thyrotropin-releasing hormone (TRH) level and TRH receptors in discrete brain regions of the rat were investigated. As shown by a specific radioimmunoassay, a single dose of morphine increased the TRH level in the septum only. At 2 h after the last dose of repeated morphine, no significant changes in the TRH level were observed. At 72 h after the last morphine injection, the TRH level was increased in the striatum and hippocampus, but remained unchanged in the nucleus accumbens and septum. A radioreceptor assay showed that acute morphine had no effect on the density or affinity of TRH receptors in the brain regions studied. In contrast, repeated morphine increased the Kd of TRH receptors in the striatum at 2 h (by ca 42%) and 72 h (by ca 26%), and in the nucleus accumbens at 72 h (by ca 26%) after the last drug injection. At 2 h after the last morphine injection, the Bmax of TRH receptors was decreased in the nucleus accumbens (by ca 41%) and unchanged in other structures, whereas at 72 h it was elevated by ca 27% and 49% in the striatum and hippocampus, respectively. The above results indicate that repeated but not acute administration of morphine leads to long-lasting, region-specific changes in both the TRH level and receptors.

The effects of repeated amphetamine administration on the thyrotropin-releasing hormone level. Its release and receptors in the rat brain

The effects of single and repeated administration of amphetamine (5 mg/kg, i.p., twice a day for 14 days) on the thyrotropin-releasing hormone (TRH) level, release and receptors in the rat striatum and nucleus accumbens were evaluated. Both treatments decreased the TRH level in those structures at 2 h after the drug injection. These effects were accompanied with elevation of the basal release of TRH from the nucleus accumbens and striatal slices at the same time point, whereas the stimulated (K+, 56 mM) TRH release was attenuated following repeated amphetamine administration. Acute amphetamine had no effect on the density and affinity of TRH receptors. Repeated amphetamine increased the Bmax of TRH receptors in the striatum (by ca 49%) and nucleus accumbens (by ca 38%) at 2 h after the last drug injection. At 72 h after the last amphetamine administration, the Bmax of the TRH receptor in the striatum was still elevated (by ca 42%), whereas in the nucleus accumbens it returned to control level. No changes in the affinity of TRH receptors following repeated amphetamine were found. The obtained results indicate that repeated amphetamine evokes long- and short-term up-regulation of TRH receptors in the rat striatum and nucleus accumbens, respectively. Furthermore, it is suggested that these changes may be an adaptive response to the amphetamine-induced alterations in the TRH tissue level and release.

The effect of p-chloroamphetamine and p-chlorophenylalanine on the level of thyrotropin-releasing hormone and its receptors in some brain structures and lumbar spinal cord of the rat

The concentration of thyrotropin-releasing hormone (TRH) and the density and affinity of TRH receptors were examined in the ventral and dorsal lumbar spinal cord, nucleus accumbens and striatum of rats with the 5-hydroxytryptamine (5-HT) nerve terminal destroyed with p-chloroamphetamine (PCA), or in animals treated with the inhibitor of 5-HT synthesis p-chlorophenylalanine (PCPA). PCA (2 x 10 mg/kg i.p., 9 and 8 d before killing) and PCPA (3 x 300 mg/kg i.p., 72, 48 and 24 h before killing)--either of them dramatically diminishing the 5-HT and 5-HIAA concentrations in all the examined structures--reduced the TRH level and increased the density of TRH receptors in the ventral lumbar spinal cord. PCPA also reduced the TRH content in the nucleus accumbens. The PCA-induced reduction in the TRH level and increase in the density of TRH receptors in the ventral lumbar spinal cord were significantly attenuated by citalopram (2 x 20 mg/kg i.p., 30 min before PCA), a selective inhibitor of 5-HT uptake. Our results constitute a further proof that coexistence of TRH and 5-HT takes place in the ventral lumbar spinal cord and then indicate that other form(s) of relationship between 5-HT and TRH may exist in some parts of the central nervous system. They also suggest that an up-regulation of TRH receptors occurs in the spinal cord as a result of TRH depletion.

The role of dopamine in regulation of thyrotropin-releasing hormone in the striatum and nucleus accumbens of the rat

The effect of alpha-methyl-p-tyrosine (alpha-MT), FLA-63, amphetamine, apomorphine and quinpirole on the concentration of thyrotropin-releasing hormone (TRH) in the striatum and nucleus accumbens was studied in rats. It has been found that the TRH content was increased in both those structures after alpha-MT, an inhibitor of tyrosine hydroxylase which reduced the concentration of both dopamine (DA) and noradrenaline (NA), but not after FLA-63, an inhibitor of DA-beta-hydroxylase which decreased the NA level without affecting DA. On the other hand, the indirectly acting dopaminomimetic amphetamine, the non-selective DA receptor agonist apomorphine, and the selective D2 receptor agonist quinpirole reduced the TRH level in the striatum, but not in the nucleus accumbens. Moreover, the decrease in the striatal peptide content induced by DA-mimetics was antagonized by the selective D2-receptor antagonist sulpiride, but not by the selective D1 receptor antagonist SCH 23390. The effect of amphetamine was not modified by the selective alpha 1-adrenoceptor antagonist prazosin. These results indicate that DA and D2 receptors play a significant role in the regulation of the striatal TRH.